The present invention relates to the measurement of the current density distribution in electron and ion beams, particularly to a technique using a modified Faraday cup to create an image of the current density of such beams, and more particularly to a system and method using a rotating modified Faraday cup in conjunction with computer tomography to determine the current density distribution in electron and ion beams.
Over the years, various apparatus have been developed for determining various characteristics of electron and ion beams, such as the beam configuration, diameter, energy peak, current density, spot size and edge width, etc. These prior approaches are exemplified by U.S. Pat. Nos. 4,336,597 issued Jun. 22, 1982 to T. Okubo et al.; 4,480,220 issued Oct. 30, 1984 to S. Isakozawa et al.; 4,629,975 issued Dec. 16, 1986 to R. Fliorito et al.; and 4,675,528 issued Jun. 23, 1987.
Electron beam machines have found wide application, particularly in the field of welding, surface modification, x-ray generation, electron beam lithography, electron microscopy, etc. With these applications has come the need for precise control of the beam focus and beam alignment, as well as a particular need for determining the power distribution in electron beams.
Reproducible electron beam processing can be made independent of the machine or the operator if the beam power distribution can be precisely controlled. Convention methods for setting the power distribution rely on the welding operator to visually focus the beam on a secondary target. The operator views the visible radiation or intensity of light given off rather than directly measuring the power distribution of the beam. This prior method is inherently imprecise, requiring significant operator experience and judgment to set the beam focus consistently. As readily seen, each operator may set the machine parameters differently due to each one's visual interpretation.
The current density distribution is influenced by many variables, including the filament design, focus setting, work distance, beam current, accelerating voltage, vacuum level, and filament alignment. Variations in these parameters may result in variations in the current density distribution of the beam, which can have a significant effect on the weld penetration, weld width, and surface quality of electron beam welded materials. Thus, it is seen that the conventional methods for setting the power distribution of electron beam machines has not been fully satisfactory. Thus, quantitative diagnostic methods such as the rotating wire device, the pinhole devices, and the modified Faraday cup (MFC) have been developed to more accurately determine the current density distribution and thereby more accurate control of the beam focus conditions. The Faraday cup approach is exemplified by U.S. Pat. Nos. 4,608,493 issued Aug. 26, 1986 to Y. Hayafuji; 4,703,256 issued Oct. 27, 1987 to Y. Hayafuji; and 5,103,161 issued Apr. 7, 1992 to J. M. Bogaty.
The rotating wire device operates by scanning a thin electrically conductive wire through the beam to sample the beam current. This early diagnostic method provided a means to measure the diameter of the beam, however, the accuracy of this method was limited by poor heat dissipation from the wire and difficulties associated with collecting back scattered and secondary electrons.
The pinhole devices provided another method for analyzing the spatial distribution of current of non-uniform electron beams using a pinhole aperture to sample the current density. By rastering the beam over a pinhole, current density measurements can be acquired at regularly spaced intervals throughout the cross-section of the beam. Therefore, the current density, and thus the power density distribution, can be determined without making any assumptions about the beam symmetry. Although this method has the ability to map the spatial distribution of power in the beam, it tends to have a high signal-to-noise ratio due to the fact that only a small percentage of the beam is transmitted through the pinhole, and inaccuracies caused by damage to the pinhole while acquiring data are not correctable.
The modified Faraday cup (MFC) devices involve sweeping the electron beam across a narrow slit. The major limitation of the MFC method is that the beam is assumed to be radially symmetric with a circular cross-section section in order to measure the current density distribution with a single scan.
When a beam with a radially symmetric Gaussian current density distribution is integrated along one dimension, as is done by the slit, the result is a one dimensional Gaussian waveform which is a good indication of the quality of the beam. However, when an irregularly distributed (non-uniform) beam current is similarly integrated, the result is an irregularly shaped waveform which, by itself, tells little about the beam's power distribution. Such non-uniform beams tend to take on an elliptical or irregular cross-section power distribution, and tend to produce non-symmetric welds or processing conditions. The geometric shape of these welds varies with the orientation of the beam with respect to the weld direction, and an accurate method for measuring the current density distribution is required in order to precisely control the welding process. Fortunately, a number of measured waveforms taken at different slit angles can be used to reconstruct the beam current density distribution using computed tomography (CT) algorithms developed for the non-destructive evaluation of solid objects, thus providing information about non-symmetric beam distributions.
Computed tomography imaging has been developed in recent years to reconstruct the interior structure of objects using x-ray data, as described in "Computerized Tomography Reconstruction Technologies", Energy and Technology Review, November-December 1990, S. G. Azevedo et al., UCRL-52000-90-11.12, pp. 18-34. In this method an object is scanned using an x-ray source and a detector or a detector array. Detected x-rays follow a line connecting the x-ray source and the detector. This ray trace is analogous to the narrow slit on the MFC. The detected x-ray intensity will vary depending on the densities of materials in the path, just as the current measured through the MFC will depend on the current density along the slit. The waveform resulting from a scan is called a projection. The object is rotated and scanned at angles between 0-180 degrees, as scanning more than 180 degrees produces redundant data. The projections are processed and used to reconstruct the beam current density distribution. The greater the number of angles, the greater the accuracy of the reconstruction. In reconstructions where fine detail of the solid object is required, increments of one degree or less may be used.
In view of the need to provide an approach for measuring the current density distribution in electron beams, it has been recognized that by combining a modified Faraday cup with the computer tomographic technique and rotating the Faraday cup, a tomographic determination of the power distribution in electron beams could be obtained, thereby resulting in the present invention. The modification of the Faraday cup (MFC) involves rotating the cup in a selected sequence, such as by a stepper motor which allows for repeated rotation of the MFC and reorientation of the MFC slit angle, with deflection coils used in sweeping the beam across the slit, thereby producing beam current waveforms which are measured with a current viewing resistor and recorded with a device such as a digital storage oscilloscope, whereafter the waveforms are reconstructed utilizing computer tomography to produce a surface plot of the power density distribution of the electron beam.